As one of its components, a volumetric flowmeter has a pump portion. The pump portion is equipped with a measuring chamber provided in a flow path and a pair of rotors causing a fixed volume of fluid to flow out for each rotation in the measuring chamber. The volumetric flowmeter can measure flow rate from rotation of the rotors. More specifically, the volume defined by the measuring chamber and the rotors is regarded as a reference volume, and it is possible to obtain flow rate from the rpm of the rotors while discharging fluid flowing into the measuring chamber according to the rotation of the rotors.
Owing to a capability of direct measurement of volume flow rate and high accuracy thereof, the volumetric flowmeter is widely used as a flowmeter for industrial and transaction uses.
As a volumetric flowmeter which accurately detects the pressure loss between the outlet and inlet of the flowmeter in order that stable flow rate measurement may be effected without being influenced by the physical values of the fluid, such as viscosity and density, and which imparts drive force from the outside to the rotors so as to keep the pressure loss at zero, making it possible to measure the flow rate at this time from the operational rpm of the rotors, there is known a servo type volumetric flowmeter as disclosed in Japanese Patent No. 3,331,212.